This invention relates to fluorescent lamps and phosphor particles. More particularly, this invention is concerned with the improvement of lumen maintenance of fluorescent lamps.
Fluorescent lamps are low-pressure mercury arc discharge devices which have electrodes at each end of an elongated glass envelope and which contain a phosphor coating on the inner surface of the glass envelope. Such lamps experience a gradual decrease in light output with increasing hours of use. A variety of factors contribute to the drop-off in light output during lamp operation. These factors include deposits of impurities from the cathode; the formation of various mercury compounds due to the bombardment of the phosphor by atoms and ions of mercury; changes in the phosphor itself; and changes in the glass envelope, particularly where it may be subject to ultraviolet radiation. The ability of such lamps to resist the drop-off in light output is generally termed lumen maintenance which is measured as the ratio of light output at a given life span compared to an initial light output and expressed as a percentage.
While the decrease in light output with time is an occurrence for all fluorescent lamps, it presents a greater problem for high output and very high output lamps than for normally loaded lamps, and for phosphors particularly susceptible to degradation in the hostile environment of the discharge.
Although all of the factors listed above can be present to a greater or lesser degree in acting to reduce light output, it is presently believed that one of the primary causes of the drop-off in light output during operation is the formation of mercury compounds, particularly on the surface of the phosphor coating.
These mercury compounds are believed to form an ultraviolet radiation absorbing film which prevents the phosphor from being sufficiently excited by the radiation from the mercury discharge to achieve maximum light output.
Various uses of alumina have been proposed to improve lamp performance. One such use involves employing a layer of alumina on the interior of the bulb wall and applying the phosphor layer thereover. Another use is the application of a thin layer of alumina over the phosphor layer.
While these techniques provide some benefit, it is believed that obtaining further increases in lumen maintenance while applying only one layer of material to the inner surface of the lamp envelope is desirable.